The present invention relates to laminated glass and the polymeric layers used therein. More particularly, the present invention relates to multilayer composite interlayers used in laminated glass wherein one of the layers is plasticized polyvinyl butyral (PVB). A key performance attribute of such laminated glass is penetration resistance which is normally determined via the 2.27 kg (5 lb.) ball drop test wherein a Mean Break Height (MBH) can be measured via the staircase or energy methods. Automotive windshields for use in vehicles in the United States must pass the minimum penetration resistance specification (100% pass at 12 feet) found in the ANSI Z26.1 code. In other parts of the world there are similar codes that are required to be met. There are also specific code requirements in both the US and Europe for use of laminated glass in architectural applications wherein minimum penetration resistance must be met.
The staircase method utilizes an impact tower from which the steel ball can be dropped from various heights onto a 30.5×30.5 cm sample. The MBH is defined as the ball drop height at which 50% of the samples would hold the ball and 50% would allow penetration. The test laminate is supported horizonally in a support frame similar to that described in the ANSI Z26.1 code. If necessary an environmental chamber is used to condition laminates to the desired test temperature. The test is performed by supporting the sample in the support frame and dropping a ball onto the laminate sample from a height near the expected MBH. If the ball penetrates the laminate, the result is recorded as a failure and if the ball is supported, the result is recorded as a hold. If the result is a hold, the process is repeated from a drop height 0.5 m higher than the previous test. If the result is a failure, the process is repeated at a drop height 0.5 m lower than the previous test. This procedure is repeated until all of the test samples have been used. The results are then tabulated and the percent hold at each drop height is calculated. These results are then graphed as percent hold versus height and a line representing the best fit of the data is drawn on the graph. The MBH can then be read from the graph at the point where the percent hold is 50%.
The energy test method determines the MBH by applying the principle of conservation of energy to a ball impacting a laminate. The difference in energy contained by the ball after penetrating through a laminate is equal to the amount of energy absorbed by the laminate. The energy of the ball as it strikes the laminate can be calculated from the drop height. The energy of the ball after it exits the laminate can be determined by measuring the velocity of the ball as it passes two magnetic field detector coils which are separated by a known, fixed distance beneath the laminate. These values can then be used to calculate the MBH.
In order to achieve acceptable penetration resistance (or impact strength) for a glass/PVB/glass laminate, it is essential for the interfacial glass/PVB adhesion levels to be maintained at about 3-7 Pummel units. Acceptable penetration resistance is achieved at a pummel adhesion value of 3 to 7, preferably 4 to 6. At a pummel adhesion value of less than 2, too much glass is lost from the sheet and glass spalling during impact as well as problems with laminate integrity (i.e., delamination) and long term durability can occur. At a pummel adhesion of more than 7, adhesion of the glass to the sheet is generally too high and results in a laminate with poor energy dissipation and low penetration resistance.
Adhesion of PVB to glass is measured using a pummel adhesion test (pummel adhesion value has no units) that is routinely used for quality control purposes in the laminated glass industry. Glass/PVB/glass laminates are prepared, conditioned to −18° C. (0° F.) and each is manually “pummeled” with a 1 pound (454 g) ball peen hammer to break the glass. All broken glass unadhered to the PVB sheet is removed. The glass left adhered to the sheet is visually compared with a set of standards of known pummel scale, the higher the number, the more glass that remains adhered to the sheet—i.e. at pummel zero no glass is adhered and at pummel 10, 100% of the glass is adhered to the sheet surface.
Another factor in addition to adhesion that is an important consideration for determining penetration resistance is the PVB film thickness in the laminate. Since a large percentage of PVB interlayer used in the production of windshields is heated and then shaped/stretched to achieve a curved color band to conform to the vehicle roof line, a combination of too high adhesion and thin PVB interlayer can also cause finished windshields to fail the required penetration resistance performance standard. Because it is not possible to reduce adhesion (and achieve acceptable penetration resistance) after windshields are laminated, windshields must be destroyed if they fail to meet the minimum requirements.
In order to make acceptable interlayer products for use in laminated glass applications, significant efforts are expended to manufacture product with consistent adhesion performance. This is done via tight manufacturing control of resin, plasticizer and other components as well as quality control evaluation of peel adhesion for each lot of interlayers that are produced. PVB interlayers are also manufactured at a controlled level of moisture since interlayer moisture has a large influence on PVB/glass adhesion levels. On the customer's side, there are numerous factors that can affect PVB/glass adhesion including glass source, glass washing, interlayer moisture content, etc. It is extremely important to maintain laminate assembly rooms and PVB blank storage rooms at controlled humidity and temperature to prevent changes in interlayer moisture content. Secondary processing operations such as PVB shaping described in a previous section can result in changes in PVB interlayer moisture content and have accompanying significant effects on PVB/glass adhesion levels and penetration resistance. It is often possible to have very different adhesion behavior on each glass surface or variable/spotty adhesion within a windshield due to glass surface cleanliness that can result in unacceptable penetration resistance and again unacceptable quality. A small percentage of windshield production must be destructively tested to assure that the manufactured product meets the targeted pummel adhesion and required MBH specifications.
Another significant performance deficiency with PVB based laminates is the effect of temperature on penetration resistance. At 0° F. the observed MBH is about 30-40% of the MBH that is achieved at 75° F.
It has been known for a long time that with proper selection of the urethane polymer structure (in particular, the soft segment portion) that the sensitivity of the penetration resistance to lower test temperatures for glass/PU/glass laminates can be greatly reduced.
Another significant use of polyurethane interlayers wherein PVB interlayers do not participate is in the fabrication of specialty laminates involving glass/polycarbonate/glass wherein penetration resistance of the glass laminate is predominately controlled by the polycarbonate component and the PU component acts primarily as an adhesive for the laminate construction. Plasticizers used in all commercially available PVB interlayers chemically attack the polycarbonate surface resulting in a crazing/haze and unacceptable quality.
Glass/PU/glass laminates typically show excellent resistance of PU/glass adhesion to high humidity and temperature in contrast to PVB based laminates.
In spite of the benefits displayed by polyurethane based laminates, such laminates have not replaced PVB based laminates because of the higher cost of polyurethane polymer. Accordingly, there exists a need in the art for an interlayer that can be used in laminated glass which minimizes the effects of temperature and adhesion on observed penetration resistance with the lower cost and other properties associated with PVB based laminates. The present invention provides a composite interlayer that incorporates many of these benefits.